cPickle.load vs. file.read+cPickle.loads on large binary files

[Peter Otten]

andrea.gavana at gmail.com wrote:

> Hi Chris,
> 
> On Tuesday, November 17, 2015 at 4:20:34 PM UTC+1, Chris Angelico wrote:
>> On Wed, Nov 18, 2015 at 1:20 AM,  Andrea Gavana wrote:
>> > Thank you for your answer. I do get similar timings when I swap the two
>> > functions, and specifically still 15 seconds to read the file via
>> > file.read() and 2.4 seconds (more or less as before) via
>> > cPickle.load(fid).
>> >
>> > I thought that the order of operations might be an issue but apparently
>> > that was not the case...
>> 
>> What if you call one of them twice and then the other? Just trying to
>> rule out any possibility that it's a caching problem.
>> 
>> On my Linux box, running 2.7.9 64-bit, the two operations take roughly
>> the same amount of time (1.8 seconds for load vs 1s to read and 0.8 to
>> loads). Are you able to run this off a RAM disk or something?
>> 
>> Most curious.
> 
> 
> Thank you for taking the time to run my little script. I have now run it
> with multiple combinations of calls (twice the first then the other, then
> viceversa, then alternate between the two functions multiple times, then
> three times the second and once the first, ...) with no luck at all.
> 
> The file.read() line of code takes always at minimum 14 seconds (in all
> the trials I have done), while the cPickle.load call ranges between 2.3
> and 2.5 seconds.
> 
> I am puzzled with no end... Might there be something funny with my C
> libraries that use fread? I'm just shooting in the dark. I have a standard
> Python installation on Windows, nothing fancy :-(

Perhaps there is a size threshold? You could experiment with different block 
sizes in the following f.read() replacement:

def read_chunked(f, size=2**20):
    read = functools.partial(f.read, size)
    return "".join(iter(read, ""))